I do not know if this answer helps you, but in my case it had something to do with server domain names and case sensitivity.
The URL we used for this working copy was
https://bhm18a.serona.org:8443/svn/nebeam/eco/branches/apple2010
instead of the correct URL
https://bhm18a:8443/svn/NeBeam/eco/branches/apple2010
Misteriously, the wrong URL worked for “check out” and “update” and browsing the respository but not for “copy” or “commit”.
Checking out a new working copy using the exact URL made the problems disappear.
(Using subversion 1.6.12 with Visual SVN Server installed on a Microsoft Windows Server)